Road surfaces are subject to varied faults that develop due to traffic and weather. For example asphalt road surfaces can develop relatively narrow depressions or ruts in the direction of traffic flow caused by heavy trucks and the like. These ruts can be relatively short and localized, or often can continue for a considerable distance, often for miles, along the road.
Asphalt road surfaces are typically laid with two passes of a paving machine that abut each other at the centerline of the road. Cracks commonly develop along the centerline where the two passes meet. Again thermal expansion and contraction due to weather extremes can cause lateral cracks that extend from the centerline to the edge of the road. A characteristic of these cracks is that the edges are depressed causing increased impact forces on the edges from traffic, and leading to further deterioration of the road surface.
At typical ambient temperatures, most asphalts are highly viscous and too stiff to apply to a roadway. To get it into a form that can be applied requires the viscosity to be reduced. This can be done by heating the asphalt, by diluting the asphalt with gasoline, kerosene or the like to make a cutback asphalt, or by making an asphalt emulsion.
Cutback asphalts harden when the diluting fluid evaporates into the atmosphere. Environmental concerns have thus made the use of such diluted asphalt undesirable. Asphalt emulsions consist of water with fine particles of asphalt dispersed in it, and thus do not present a similar environmental risk. Chemical solutions called emulsifiers give the asphalt particles the ability to stay suspended in water, and such asphalt emulsions are well known and are formulated to have a desired viscosity suitable for applications at ambient temperature. Polymers are commonly added to the asphalt emulsion to increase cohesion of the asphalt to aggregate.
Conventional road surface maintenance procedures include using slurries in various applications. A slurry is a mixture of fine aggregate, asphalt emulsion, and fillers that is cold mixed at ambient temperature. The asphalt emulsion serves as a binder, holding the particles of the aggregate together and binding the slurry to the surface on which it is applied. The fillers are added to stabilize or modify the characteristics of the slurry. Once all the ingredients are mixed together, a break time begins to count down. The slurry “breaks” when the asphalt separates from the water in the asphalt emulsion, either by evaporation or with the aid of emulsifiers that react with the aggregate. Adding Portland cement as a filler also reduces the break time, and further gives the slurry a creamy consistency that aids in even spreading. After the slurry breaks, it cures or hardens to a satisfactory hardness to support traffic.
When slurries are first mixed, they are in liquid form of varying viscosities according to the ingredients and thus readily flow into cracks and depressions and leave a smooth surface. As time passes the slurry breaks and then cures. The slurry must therefore be placed in position on the road surface within that period of time after mixing where it is liquid and prior to breaking. Water and other materials are added to the slurry to provide the desired viscosity and break time.
Slurries are commonly used to spread in a relatively thin layer across the surface of a road to seal the road, and restore a skid resistant surface. Considerable care must be taken with the aggregate used in a slurry. The aggregate particles must be sourced from crushed rock in order to provide particles with jagged edges so that they bind to the asphalt emulsion in a satisfactory manner. Fine particles from a pit or like natural source typically have relatively smooth surfaces that have been polished by wind and water over considerable time, and thus are not generally satisfactory for use in a slurry.
Such sealing slurries are commonly mixed in batches and the ingredients chosen to provide a long break time, an hour or more, so that the operators will have time to spread the slurry on the area, or a number of different areas, where it is required. The long break time somewhat delays the resumption of traffic flow on the road, however sealing a whole lane surface is an operation that is not required on a regular basis and so the delay in traffic resumption is considered acceptable.
Slurries have also been developed that are suitable for filling long and relatively deep ruts in road surfaces, as well as re-surfacing whole road surfaces, using a process commonly referred to as microsurfacing. Microsurfacing slurries have a very short break time, typically minutes, so that the thicker layer of slurry will harden quickly and traffic delays will be minimized. In order to mix and apply microsurfacing slurries, a mobile vehicle with a continuous mixing apparatus is used that carries the ingredients and mixes and applies the slurry as the apparatus moves along the rut. The apparatus is also configured to load aggregate and other ingredients from transport vehicles as it moves along the road so that the slurry can be laid down in long continuous stretches to maintain a smooth, uninterrupted surface.
The International Slurry Surfacing Association provides two guideline publications on their web-site at www.slurry.org entitled Recommended Performance Guidelines for Emulsified Asphalt Slurry Seal Surfaces 2003, and Recommended Performance Guidelines for Polymer Modified Micro-Surfacing 2003.
Localized depressed areas such as lateral depressed cracks and localized ruts are typically present at fairly widely spaced intervals along a road. For repairing localized depressed areas, cold mixed slurries have not been used. In repairing such cracks it is desired to use a material that remains liquid for a time long enough for the crew to move from one crack to the next some distance down the road. Each of a plurality of depressed lateral cracks requires a relatively small amount of material to fill it, and so a hopper and spreader box mounted on a loader vehicle can generally hold enough material to fill several cracks. Once laid it is desired to have the material set or harden very quickly so that traffic flow can be resumed. Thus it is preferred to use a hot mix sulfur asphalt material. This hot mix is mixed with a heated mixer, and then typically placed in a heated hopper on a loader vehicle and carried to the cracks and applied as the repair material. When maintained at a high temperature in the heated loader hopper, the hot mix stays liquid and flows readily. When applied to the road surface in a relatively thin layer, the material rapidly cools and hardens so that it is ready for traffic almost immediately after it is laid. Such hot mixes are also commonly used to patch depressed centerline cracks, as well as localized depressed areas.
Application of the repair material to the road typically uses essentially the same process for sealing slurry, microsurfacing slurry, and hot mix. The material flows into a spreading box that lays the material on the road surface as the spreading box is moved forward. The spreading box maintains close contact with the road surface on each side of the rut or crack and provides a straight edge along the back from one side to the other to screed or level the material as the box moves forward. In a continuous process like microsurfacing, the spreader box is dragged along behind the mixing vehicle and fed directly from the mixer. When applying a sealing slurry the spreader box is generally the same width as a road lane and the box is dragged behind a vehicle carrying a batch of sealing slurry. In a hot mix batch process, the spreader box is typically mounted to the underside of a heated hopper or bucket mounted on a loader vehicle. The batch of hot mix is placed in the hopper where an agitator maintains the uniformity of the mix, and flow to the spreader box underneath is controlled with a gate. The operator moves from crack to crack, dragging the hopper and box along the crack and controlling the amount of material released into the box so that the box is substantially empty when it reaches the end of the crack, and the hot mix is feathered out to the end of the crack at the edge of the road.
The conventional hot mixes used to patch localized depressed areas comprise asphalt oil, sand, and sulfur. The asphalt oil is typically heated to about 325° C. and the sand is heated to about 350° C. Conveniently natural sand, suitably washed or screened, can be used instead of crushed sand since the asphalt oil adheres better when heated rather than being carried in an emulsion. Sulfur is added in order to hasten hardening. Without sulfur, it could take 20 minutes or more for the mix to cool and harden, while with added sulfur hardening is very quick, so that traffic may be resumed almost immediately in many cases. Where the depressed areas are deeper however, it is generally desirable to delay traffic somewhat to allow the thicker mix to harden prior to traffic resumption.
The presence of sulfur in the hot mix causes the mix to give off malodorous fumes, and health concerns arise concerning workers exposed to sulfur fumes. There are as well environmental concerns respecting the use of sulfur on roads where it can be washed into surrounding soil by rain. As well heating the mixer and hopper requires combustion of fuels like propane which are inherently hazardous. Equipment and fuel for heating the hot mix also adds considerably to the cost of patching lateral depressed cracks.